The present invention relates to apparatus and methods for primary power distribution and, more particularly, to apparatus and methods for high-power power distribution using solid state power controller (SSPC) technology.
In existing aircraft primary power distribution systems, electromechanical relays/contactors, along with circuit breakers, are exclusively used for load, feeder, bus tie and power source controls. These electromechanical contactors are generally large, heavy and expensive, and have a limited contact cycle life due to arcing, wear, and metal degradation.
SSPC technology is gaining acceptance as a modern alternative to electromechanical contactors due to its high reliability, “soft” switching characteristics, fast response time, and ability to facilitate advanced load management and other aircraft functions. A typical SSPC mainly comprises a solid state switching device (SSSD), which performs the fundamental power on/off switching, and a SSPC processing engine, which is responsible for SSSD on/off control and feeder wire protection. While SSPCs with current rating less than 15A have been widely used in aircraft secondary distribution systems, their applications in aircraft primary distribution systems still face strong technical challenges.
Other than excessive power dissipation, voltage drop, and leakage current associated with SSPCs with high power ratings using existing SSSDs, predominantly metal-oxide-semiconductor field-effect transistors (MOSFETs) or MOSFETs in combination with insulated gate bipolar transistor (IGBTs), the “fail shorted” nature of SSSD also becomes a critical concern for the certification of the SSPC technology for commercial aircrafts.
In order to meet the safety and reliability requirements enforced by the certification authorities, a secondary protection mechanism, typically a fuse, has to be included in series with the SSPC. Such a protection mechanism is often sized to provide the same level of wire protection, in terms of the energy rating, which could easily interact with SSPC's trip mechanism. In addition, leakage current through off SSSDs may pose a safety issue when performing maintenance on downstream loads. Conventional solutions introduce an SSPC output clamping circuitry.
In AC applications, the basic SSSD configuration is usually formed by two MOSFETs (or other field effect transistors (FETs)) connected “back to back” to facilitate the symmetrical bi-directional power switch functions. Any one of such FET devices failing shorted when the SSPC is in the off state will result in a rectified DC power supplied to its controlling load or multiple loads, causing unexpected load behavior or potential damage, which cannot be effectively resolved by use of a fuse in series.
Various efforts have been made in search of alternative semiconductors for use in the high power SSSD to reduce the power dissipation. GaN based high electron mobility transistors (HEMTs) (e.g. GaNpowIR) and SiC based vertical-junction FETs (VJFETs) are considered two most promising candidates in terms of both the economic viability and their superior performance over the conventional Si based SSSDs. However, their “normally on” characteristics prevent them from being directly applied to power distribution system, due to safety concerns.
As can be seen, there is a need to provide a practical, certifiable solution for the SSPC to be used in aircraft primary power distribution systems. There is also a need to provide such a solution, which may result in reduced power dissipation, improved reliability and fault current handling capability, and smaller component volume.